This invention relates to the art of video signal processing and, more particularly, to clamping a nonpicture portion of a video signal to a desired reference level.
The invention has particular application to clamping the back porch portion of a video signal to a desired reference level and will be described with particular reference thereto; although, it is to be appreciated that the invention may be applied to other nonpicture portions of the video signal, such as the front porch and the sync tip.
Video signal clamping is a known technique employed in video signal processing. A typical video signal includes predetermined nonpicture portions as well as a picture portion. These predetermined nonpicture portions include the front porch, the sync tip and the back porch portions. These portions are intended to be at specific voltage levels with the front porch and back porch portions straddling the sync tip portion and being of essentially the same value. The sync tip portion is normally of a different value, usually negative with respect to the front porch and back porch portions. It is known that a typical video clamp circuit operates by identifying one of these nonpicture portions of the video signal which recurs and should be of a constant voltage or a known AC component superimposed on a constant voltage and then clamps this selected nonpicture portion from the video signal to a desired reference level.
If noise is present on a nonpicture portion of the video signal at the time clamping takes place, then objectionable picture streaking may take place. It is known in the art to employ circuitry for minimizing this picture streaking, caused by clamping to noise, and such a known technique takes the form as that illustrated and described in the U.S. Pat. No. to Nakamura, 4,516,042. Nakamura discloses a system in which a back porch sampler samples the back porch portion of the video signal and applies this to a variable time constant amplifier and supplies an offset voltage to a video amplifier which also receives the video signal so that the output of the video amplifier is driven toward the desired voltage level. A noise detection circuit responds to the noise present on the back porch portion sample and develops a control signal to vary the time constant of the variable time constant amplifier.
It is conventional to employ a video clamp circuit which includes a resistor and a capacitor in series circuit with a switching means to a reference level. The switch is periodically closed for a fixed period of time during a predetermined nonpicture portion of the video signal so as to clamp the video signal to the reference level by way of the RC circuit. Nakamura, supra, employs this concept but varies the time constant of the RC circuit as a function of measured noise. The switch closure time is not changed.
The present invention is directed toward a similar circuit employing an RC series circuit for clamping an input video signal to a desired reference level by way of a switch each time the switch is closed. However, contary to Nakamura, the present invention varies the switch closure time, and not the RC time constant, as a function of measured noise.